1. Field of the Invention
The invention relates to methods and systems for controlling the operation of xe2x80x9clean-burnxe2x80x9d internal combustion engines used in motor vehicles to obtain improvements in vehicle fuel economy.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (8), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as NOx, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of engine-generated constituent gases, such as HC, CO and NOx, that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and release previously stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
Under one prior art approach, the duration of any given lean operating excursion (or its functional equivalent, the frequency or timing of each purge event) is controlled based upon an estimate of how much NOx has accumulated in the trap since the excursion began. Specifically, a controller accumulates estimates of feedgas NOx over time to obtain a measure representing total generated NOx. The controller discontinues the lean operating excursion when the total generated NOx measure exceeds a predetermined threshold representing the trap""s nominal NOx-storage capacity. In this manner, the prior art seeks to discontinue lean operation, with its attendant increase in engine-generated NOx, before the trap is fully saturated with NOx, because engine-generated NOx would thereafter pass through the trap and effect an increase in tailpipe NOx emissions.
Unfortunately, empirical evidence suggests that the instantaneous storage efficiency of the trap, i.e., the trap""s instantaneous ability to absorb all of the NOx being generated by the engine, rarely approaches 100 percent. Indeed, as the trap begins to fill, the instantaneous storage efficiency of the trap appears to decline significantly, with an attendant increase in the amount of NOx being exhausted to the atmosphere through the vehicle""s tailpipe. While increasing the frequency of the purge events may serve to maintain relatively higher trap storage efficiencies, the fuel penalty associated with the purge event""s enriched air-fuel mixture and, particularly, the fuel penalty associated with an initial release of oxygen stored previously stored in the three-way catalyst during lean engine operation, would rapidly negate the fuel savings associated with lean engine operation.
Moreover, under certain engine operating conditions, for example, under high engine speed and high engine load, the NOx generation rate and correlative exhaust flow rate through the trap are both so high that the trap does not have an opportunity to store all of the NOx in the exhaust, even assuming a 100 percent trap storage efficiency. As a result, such operating conditions are themselves typically characterized by a significant increase in tailpipe NOx emissions, notwithstanding the use of the NOx trap.
For a majority of motor vehicles, the effectiveness of a given method and system for controlling tailpipe NOx emissions is generally measured by evaluating the vehicle""s performance in a standardized test under the Federal Test Procedure (FTP), in which the vehicle is operated in a prescribed manner to simulate a variety of engine operating conditions, at a variety of different engine-speed and engine-load combinations. A graphical illustration of the various engine speed/load combinations achieved during the FTP City Driving Cycle is depicted as Region I in FIG. 5, while the various engine-speed and engine-load combinations achieved during the FTP Highway Driving Cycle are depicted in FIG. 5.
During either FTP test, vehicle NOx emissions, as measured by a NOx sensor, are accumulated over the course of a thirty-minute test period. The vehicle is deemed to have passed the test if the accumulated value of tailpipe NOx, in grams, does not exceed a prescribed threshold amount. Often, the prescribed threshold amount of permissible NOx emissions under the Highway Driving Cycle is characterized as a multiple of the prescribed threshold amount for the City Driving Cycle.
The NOx emissions of certain other motor vehicles, such as heavy trucks, are measured using another approach, wherein the vehicle""s engine is independently certified on a dynamometer, with the engine""s instantaneous NOx emissions thereafter being normalized by the engine""s peak horsepower, in grams per horsepower-hour. In either event, such emissions standards are said to be xe2x80x9cscalar,xe2x80x9d i.e., fixed or static, rather than dynamic.
Significantly, it has been observed that, while the FTP City and Highway Driving Cycles include the vast majority of operating conditions over which a given motor vehicle is likely to be operated, the Cycles themselves are not necessarily representative of the manner in which most vehicles are operated. For example, it is generally true that an engine generates increased NOx emissions under operating conditions characterized by increased engine speeds and increased engine loads. Thus, each FTP cycle necessarily permits its relatively lower NOx-generating operating conditions to offset its relatively higher NOx-generating operating conditions, with a vehicle xe2x80x9cpassing the testxe2x80x9d so long as the average generated NOx does not rise to the level at which the total generated NOx exceeds the prescribed threshold after thirty minutes.
In contrast, in xe2x80x9creal worldxe2x80x9d operation, a given engine operating condition, such as a xe2x80x9chighway cruisexe2x80x9d operating condition characterized by substantially-higher instantaneous rates of NOx generation, may continue unabated for substantial periods of time. Such continued operation of the engine, even at an engine speed/load falling within Region I or Region II of FIG. 5, is properly characterized as being xe2x80x9coff-cycle.xe2x80x9d Similarly, certain circumstances, such as the towing of a large trailer, or operation of the vehicle at relatively higher altitudes, may push the operating point of the engine fully outside of Regions I and II. Engine operation under these circumstances (with engine speed/loads falling in the area generally depicted as Region III in FIG. 5) are likewise properly characterized as being xe2x80x9coff-cycle.xe2x80x9d And, because off-cycle operation may constitute a substantial portion of any given driving session, the FTP cycles do not necessarily predict the likely real-world emissions of a given vehicle.
Therefore, a need exists for a method and system for controlling the operation of a xe2x80x9clean-burnxe2x80x9d internal combustion engine which seeks to regulate all vehicle NOx emissions, including xe2x80x9coff-cyclexe2x80x9d NOx emissions.
In accordance with the invention, a method is provided for controlling the operation of an internal combustion engine in a motor vehicle, wherein the engine generates exhaust gas including NOx, and wherein exhaust gas is directed through an exhaust gas purification system including a NOx trap before being exhausted to the atmosphere. Under the invention, the method includes determining a current rate at which NOx is being exhausted to the atmosphere; determining a threshold rate for maximum permissible NOx emissions as a function of at least one of the group consisting of an engine speed, a vehicle speed, an engine brake torque, an engine manifold air pressure, and a throttle position; and determining a differential rate based on the current rate and the threshold rate. The method further includes selecting a restricted range of engine operating conditions based on the differential rate, wherein the restricted range of engine operating conditions is characterized by a plurality of air-fuel ratios, each of the plurality of air-fuel ratios being not leaner than a near-stoichiometric air-fuel ratio. By way of example, in an exemplary embodiment, the restricted range of engine operating conditions is selected when an accumulated measure based on the differential rate exceeds a near-zero threshold value.
In accordance with a feature of the invention, determining the current rate is achieved either by sampling the output signal generated by a NOx sensor positioned downstream of the NOx trap or, alternatively, calculating the current rate by determining a generation rate representative of the NOx content of the exhaust being instantaneously generated by the engine, determining a storage rate representative of an instantaneous rate at which NOx is being stored by the trap, and subtracting the storage rate from the generation rate.
In accordance with another feature of the invention, an exemplary method for practicing the invention further includes calculating a cumulative amount of NOx stored in the trap using the current rate; and selecting the first region of engine operating conditions when the cumulative amount exceeds a trap capacity value. Preferably, the method further includes determining the trap capacity value as a function of at least one of the group consisting of a trap temperature, a trap sulfation level, and an air-fuel ratio.
Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.